Seems the category selector does have problems with lazy loaded categories when the tag selector is used as well. For example when you open Topics tagged official the appearance depends on whether you have added plugin and extras to your sidebar.
When you have added both everything works fine
When you remove extras from the sidebar the subcategory selector is missing
And when you also remove plugin from the sidebar the category dropdown does not show the selected category anymore (and the category banner doesn’t show the category)
When you click ‘Hot’ or ‘Top’ from there, Discourse disregards the category filter. For instance, instead of transitioning from ‘Latest’ at https://meta.discourse.org/tags/c/plugin/22/official/l/latest to ‘Top’ at https://meta.discourse.org/tags/c/plugin/22/official/l/top, you are redirected to https://meta.discourse.org/tag/official/l/top.